The recent analysis of the B. burgdorferi genome revealed the existence of multiple paralogous gene families encoded both, in the plasmids and the chromosome. The bmp (borrelia membrane protein) locus in the megabase chromosome of Borrelia burgdorferi is one such gene family that includes four homologous genes which are organized in the following order bmpD-bmpC-bmpA-bmpB. The BmpA (or P39) protein of this family is a well-characterized immunodominant antigen of B. burgdorferi and serves as a marker of active infection. Within this locus, the genes bmpA and bmpB constitute an operon, in contrast to bmpC and bmpD, which are monocistronic. Previous studies revealed that in the B. burgdorferi strain JD1, bmpA , bmpB and bmpD, but not bmpC, were expressed in culture. We extended these studies to examine the expression of the Bmps during infection of the vertebrate host. BmpA, BmpC and BmpD proteins were overexpressed as fusion proteins in Escherichia coli and purified. BmpB prot ein could not be successfully expressed in E. coli for unknown reasons. The purified BmpA, BmpC and BmpD proteins were then used to detect the presence of monospecific antibodies, by Western blotting, in the serum of animals experimentally-infected with B. burgdorferi. Serum from three rhesus monkeys with either an early infection {week 5 post-infection (p. i.)} or a late infection (week 24-27 p.i.), and serum (6 weeks p. i.) from two infected mice, reacted only to BmpA. In summary, although cultured spirochetes express BmpA, BmpB and BmpD proteins, in the vertebrate host, only BmpA and likely BmpB (because it is located in the same operon) appear to be expressed. The data suggest that BmpA/B, BmpC and BmpD, are functionally distinct proteins. FUNDING Base Grant PUBLICATIONS Ramamoorthy R and Philipp MT. Differential expression of Borrelia burgdorferi proteins during growth in vitro. Infect Immun 66:5119